The present invention relates to a process for purification of calcium carbonate-rich by-product generated in an industrial process, particularly in nitrophosphate plants in the fertilizer industry, and converting the by-product to finely divided calcium carbonate.
The purification is carried out by thermal treatment of wet calcium carbonate-rich by-product, as available from a nitrophosphate plant. The thermal treatment is applied at a specific temperature, so as to remove moisture and volatile matter, and to decompose or sublime impurities present such as ammonium and nitrate salts. Special care is taken not to decompose the calcium carbonate. In addition, the finely divided calcium carbonate obtained may be coated by using an emulsion or solution of a fatty acid or of a salt of a fatty acid.
Calcium carbonate is used in the manufacture of paper, rubber, plastics, glass, textiles, putties, chalks, sealant, adhesives, paints, inks, varnishes, food, cosmetics, dentifrices, chemicals and pharmaceuticals.
Calcium carbonate produced according to the process of this invention is particularly suitable for use as filler in rubber, paint, plastics and PVC.
Several physical and chemical processes have been suggested for removal of impurities from waste calcium carbonate. Physical processes requires drying and grinding, to a fineness sufficient that impurities can be removed by screening, classification, magnetic separation, hydrocyclone separation or floatation. The disadvantages associated with these physical methods of removing impurities are the requirement of specialized machines/devices and the maintenance of the specialized machines and devices. Moreover, these processes are complex and difficult to administer. Other disadvantages of physical processes include unpredictable process efficiencies, variation in the quantities of impurities removed and the expenses associated with employing such physical techniques for removing impurities.
Leaching or bleaching the dried and ground calcium carbonate with chemicals such as halogen, oxygen-containing acids of a halogen or salts thereof, ozone, hydrogen peroxide etc. are described in British patent No. 1285891 (1972) as a chemical purification process. Disadvantages of chemical processes include the requirement of special chemicals, and the number of unit operations to perform the treatments. In addition, the use of a single chemical may not be suitable to remove all the impurities. Removal and separation of excess chemicals remaining after the chemical process is another disadvantage.
Another previously known and widely used method for the treatment of waste containing calcium carbonate, generally known as lime sludge, is re-burning. Reburning consists of burning within the kiln and removing the burned lime after cooling in a material cooler fitted immediately after the kiln. The burned lime reaches a temperature of 1000-1200xc2x0 C. during the treatment in the kiln. Many technological improvements have been made in kiln design. Such improvements include incorporating a drier crusher, e.g., a hammer mill type crusher, which is mounted before the kiln and cyclone separators. Such a plant, which is also known in other contexts, is described by L. Lenado and Ramon Aguillon (EP 0041113, 1982). The Lenado process is associated with many disadvantages, such as the incorporation of a new machine, the maintenance of the new machine, necessity of special steel which can withstand high temperature, high dust loss, the requirement of cyclone separators and caking in ducts due to the dust from the kiln being sticky in the temperature range 600-700xc2x0 C. The product obtained is calcium oxide which must be further processed by hydration and carbonation, to produce industrially useful calcium carbonate. Otherwise, as the Lenado process is practiced, the product is recycled in a causticizing process in producing paper pulp.
Mattelmaki discloses a process (U.S. Pat. No. 5,110,567) wherein the dryer crusher has been replaced with a pneumatic drying device, which lowers the final temperature of the exhaust gas. However, this process has the main disadvantage that the material processed is not free flowing. The material demonstrates a dry sticking tendency which often causes ring formation in the preheating zone of the rotary kiln. In addition, this process produces calcium oxide and not calcium carbonate.
Mehaffey et al, (U.S. Pat. No. 4,892,649) disclose a process for purifying calcium carbonate ore by the removal of silicate impurities from the ore using reverse floatation. The process achieves high yields and low acid insoluble content in the calcium carbonate product by employing novel collectors which comprise organo-nitrogen compounds such as quaternary ammonium compounds. The disadvantages of this process are that a floatation unit is required which must be operated continuously. Disposal of separated impurities and removal of unused quaternary compounds raises concerns about pollution resulting from this process.
Theil Jorgen (U.S. Pat. No. 5,711,802) discloses a method and a processing plant for treatment of lime sludge formed by the caustizing process during the manufacture of paper pulp. According to the Theil method, the lime sludge is dried, pulverized and then preheated in a kiln at a predetermined temperature in the range 400-600xc2x0 C. to obtain dry and poulvurized calcium carbonate. The dried material thus obtained is further calcined/burned in any type of suitable kiln to produce quick lime which in turn can be reused in causticizing process. The drawbacks associated with this invention are that the lime sludge must be dried, pulverized and preheated prior to calcination. The product obtained is calcium oxide and therefore can not be used in rubber, paint, plastics and PVC.
Drummand (U.S. Pat. No. 5,690,897), describes a method for purification of calcium carbonate by lowering the iron content. The method involves addition of an iron chelating agent to an aqueous calcium carbonate slurry, reducing the pH of the slurry by employing CO2 containing gas, while maintaining the slurry in a temperature range of 20 to 100xc2x0 C. The resulting slurry is stirred, filtered, washed and dried. The disadvantage of this invention is that it is limited to the removal of iron impurities. Moreover, continuously passing CO2 gas and maintaining pH at constant value, while simultaneously maintaining the temperature is both difficult to accomplish and uneconomical.
Price et. al. (U.S. Pat. No. 4,793,985, disclose a process for producing ultra fine ground calcium carbonate in which a dry feed limestone is slurried to a solids content of 15 to 60% by weight. The slurry is dispersed with an organic polyacrylate (DISPEX N40) and other macromolecular dispersing agents. The dispersed slurry is fed to an attrition mill where it is bead ground to produce a material wherein 50-70% of the particles are less than 2 micron. The ground calcium carbonate is classified in a centrifuge into a product stream having from 70-99% of its particles less than 2 microns and an underflow stream of larger particles. The product is then treated with ozone which serves not only to increase the brightness of the product but to return the slurry to a non-dispersed state. The calcium carbonate slurry is then dewatered to produce a product containing 58-69% by weight solids, which is suitable for preparing a higher solids content slurry or for spray drying. The drawbacks associated with the Price process is that, ozone being a rare gas, is utilized for merely for improving brightness. Use of ozone also adds to the production cost.
Bunger et al. (U.S. Pat. No. 5,846,500) disclosed a process for the purification of calcium hydroxide using a highly dilute aqueous solution for dissolution of calcium hydroxide followed by settling and filtration for the removal of impurities. The Bunger process is applied particularly to removing impurities from hydrated carbide lime, which is used subsequently as a feed stock for a high value calcium carbonate. Precipitated calcium carbonate is formed by reaction of the purified calcium hydroxide solution with CO2 in a plug-flow reactor system. The drawbacks of this invention is that due to the very low water solubility of calcium hydroxide high dilution is required to dissolve all of the calcium hydroxide. Moreover, settling and filtration steps involving large volumes of hydroxide solution becomes difficult and requires settling tanks and filtration units which serve to make the process economically unattractive.
It is evident from the above facts that the need exists for development of a new process for the generation of calcium carbonate from industrial by-products rich in calcium carbonate.
The work described in the present invention has been carried out to purify the calcium carbonate rich by-product generated in a nitrophosphate plant in a fertilizer or other industry so as to convert the by-product to an industrially useful value-added product.
The present invention provides a process for purification of calcium carbonate-rich by-product generated in industrial processes, which process obviates the drawbacks detailed above.
The present invention also develops an alternative source to limestone for producing finely divided calcium carbonate useful for commercial applications.
The present invention also provides a continuous process for the purification of calcium carbonate-rich by-product having a fine particle size from 20 to 150 microns as generated in a nitrophosphate fertilizer plant, in order to minimize the pollution caused by such solid waste and thereby improve the quality of the environment.
The present invention also provides a thermal treatment method for purification of calcium carbonate-rich by-product generated in a nitrophosphate fertilizer plant.
The methods of the present invention produce purified and coated finely divided calcium carbonate useful as filler in paints, plastics, rubber and PVC by profitably
The methods of the present invention produce purified and coated finely divided calcium carbonate useful as filler in paints, plastics, rubber and PVC by profitably utilizing the calcium carbonate rich by-product generated in a nitrophosphate fertilizer plant.
The present invention also provides pollution abatement measures for nitrophosphate fertilizer plants and a calcium carbonate-rich by-product treatment unit.
In one embodiment of the invention, there is provided a process for the production of finely divided calcium carbonate from a calcium carbonate-rich by-product, the claimed process comprising:
(a) continuously feeding a wet calcium carbonate-rich by-product into a rotary dryer to remove moisture and volatile matter, and to decompose or sublime ammonium compounds present as impurities in the by-product, to generate a rotary dryer treated material:
wherein the calcium carbonate-rich by-product:
contains up to 25% moisture,
has a particle size from 20 to 150 microns, and
is fed to the rotary dryer at a feed rate from 5 to 20 kg/h using a screw feeder with a rotation speed from 10 to 40 revolutions per minute; and
wherein the rotary dryer
is preheated and maintained at a temperature from 250 to 400xc2x0 C., preferably from 250 to 350xc2x0 C.;
has an angle of inclination from 1.0 to 1.4 degrees; and
has a rotation speed from 1 to 4 RPM, preferably from 2 to 3 RPM;
(b) routing water vapor, volatile matter, and ammonia gas liberated from the calcium carbonate-rich by-product in the rotary dryer to a water tank for scrubbing;
wherein the rotary drum heater is preheated and maintained at a temperature from 500 to 600xc2x0 C. has an angle of inclination from 1.0 to 1.4 degrees, and has a rotation speed from 1 to 4 RPM;
(d) after a residence time in the rotary drum heater from 30 to 90 minutes, preferably from 30 to 45 minutes, outputting the rotary drum heater treated material to a material cooler to cool the rotary drum heater treated material to near ambient temperature;
(e) outputting the cooled material from the material cooler at an outlet rate from 2 to 20 kg/hour, preferably from 5 to 15 kg/hour, which outlet rate is synchronized with the outlet rate for material output from the rotary drum heater;
(f) grinding the cooled material in a pin/attrition mill to a particle size from 1 to 20 microns;
(g) dispersing the ground material in a slurry tank to generate a slurry having from 40 to 55% solids content;
(h) heating the slurry to a temperature less than 95xc2x0 C. with agitation;
(i) adding to the heated slurry a hot emulsion or solution of fatty acid derivatives or salts of fatty acid derivatives to form a mixture, wherein the fatty acid derivatives or salts of fatty acid derivatives have a carbon chain length from 16 to 20 carbons, and are added to the slurry in an amount sufficient to obtain a final product comprising from 3 to 4% of the fatty acid derivative or fatty acid derivative salt;
(j) allowing the mixture to cool to a temperature less than 50xc2x0 C.;
(k) removing water soluble components from the mixture to generate a product; and
(l) drains and pulverizing the product to obtain finely divided coated calcium carbonate.
In one sub-embodiment of the invention, the screw feeder which feeds the calcium carbonate-rich by-product to the rotaty dryer in step (a) is rotated at a rate of 5 to 20 RPM.
In another sub-embodiment of the invention, the rotary dryer in step (a) is heated to a temperature from 260 to 400xc2x0 C.
In another sub-embodiment of the invention, the rotary dryer in step (a) has an angle of inclination from 1.08 to 1.4 degrees
In one sub-embodiment of the invention, the rotary drum heater in step (c) is preheated and maintained at a temperature from from 550 to 650xc2x0 C. In another sub-embodiment of the invention, the rotary drum heater in step (c) is preheated and maintained at a temperature from from 500 to 600xc2x0 C.
In one sub-embodiment of the invention, the outlet rate of material from the rotary drum heater in step (e) is obtained by rotating a jacketed screw conveyer type material cooler at a rate of 5 to 15 RPM.
In one sub-embodiment of the invention, the hot emulsion or solution of fatty acid derivatives or salts of fatty acid derivatives in step (i) is from 70 to 90xc2x0 C. In another sub-embodiment of the invention, the emulsion or solution of fatty acid derivatives or salts of fatty acid derivatives in step (i) is from 85 to 95xc2x0 C.
In one sub-embodiment of the invention, the hot emulsion or solution of fatty acid derivatives or salts of fatty acid derivatives in step (i) is added to the slurry in an amount sufficient to obtain a final product comprising from 3 to 4% of the fatty acid derivative or fatty acid derivative salt.
In another sub-embodiment of the invention, the hot emulsion or solution of fatty acid derivatives or salts of fatty acid derivatives in step (i) is added to the slurry in an amount sufficient to obtain a final product comprising from 2 to 3.5% of the fatty acid derivative or fatty acid derivative salt.
In another sub-embodiment, the material in step (f) is ground in the pin/attrition mill to a microparticulate size of 10 to 20 microns.
In another sub-embodiment of the invention, the slurry in step (h) is heated to a temperature from 85 to 95xc2x0 C.
In another sub-embodiment of the invention the temperature of the cooled material in step (d) is less than 35xc2x0 C.
In another sub-embodiment of the invention, the mixture of the slurry of finely divided calcium carbonate and the hot emulsion or solution of fatty acid derivatives or salts of fatty acid derivatives in step (j) to a temperature from 20 to 45xc2x0 C.
Heat treatment, as described in the present invention, significantly purifies the calcium carbonate-rich by-product generated in a nitrophosphate fertilizer plant to generate purified calcium carbonate. The calcium carbonate-rich by-product, having a moisture content up to 25%, can be fed at suitable rates through the system without adversely affecting the quality of the end product. In addition, the cooler outlet material can be ground to obtain the fineness as required for end applications. The process of coating the pulverized/finely ground material as applied herein provides a means of making the material suitable for rubber, paint, plastics and PVC.
In addition, the process of the present invention prevents air pollution by avoiding dusting with the use of indirect heating and by scrubbing the liberated ammonium and nitrogen compounds in water. More-over, the process remarkably increases whiteness/brightness of the product.